As percutaneous, transluminal coronary angioplasty (PTCA), one understands a method which is widely employed in the clinic in which the critical constriction of a coronary vessel (coronary stenosis) is mechanically dilated by a catheter and therewith made free again. To this day, the Achilles heal of PTCA is restenosis. This appears after weeks or months after successful PTCA in 12-43% of all treated patients (Lange, R.A. et al., Southwestern Internal Medicine Conference: Restenosis: The Achilles heal of coronary angioplasty, Am. J. Med. Sci., 265-274, 1993). A renewed PTCA or a bypass operation is the necessary consequence.
In the final analysis, the exact mechanism of restenosis is unexplained. However, of importance is the initial mechanical stretching stimulus in which, aside from destruction of the endothelial cells, tearing occurs in the arteriosclerotic wall region with simultaneous deposition of fibrin and blood platelets. The blood platelets on their part release vessel constricting and proliferation increasing metabolic products which veil the growth inhibiting factors of the smooth vessel musculature. A hyperplasty of the muscle media, i.e. restenosis, is the result.
Various strategies for the treatment of restenosis were examined in clinical studies in the past. Concerning pharmacological therapy, neither the use of anti-thrombotic agents (aspirin, heparin), antispastic agents (nifedipin, diltazem) or anliproliferative agents (colchicine) nor a lipid level sinking therapy had a favorable influence on the development of the restenosis. The development of new catheters for the minimalisation of tissue trauma (laser angioplasty, atherectomy catheter) has also brought no success in the prevention of restenosis.
There is no therapy for restenosis at the present. The reason is probably because the effective levels in blood after systemic application of a drug are too low to be able to be locally therapeutically effective.
Nitrogen monoxide (NO) is formed in mammalian cells from the amino acid L-arginine through the mediation of the enzyme NO synthase (NOS). NO is an important messenger substance and/or signal molecule in the human body which mediates a multitude of physiological and pathophysiological effects. In the central nervous system, NO is probably involved in the regulation of integrative performance, i.e. memory functions. In the gastrointestinal tract, an involvement in peristalsis is suspected. The NO formed by macrophages is capable of killing bacteria and parasites. Within the heart-circulatory system, NO is formed from the endothelial cells where it carries out two important functions. In the direction of the luminal side of the vessel it inhibits platelet aggregation and is therewith jointly responsible for the antithrombogenic properties of the vessel inner wall. On the side away from the lumen NO relaxes the smooth musculature and carries out a long-term proliferation inhibiting effect. An abolition of the endothelial NO, for example by injury of the blood vessel endothelium, leads to high blood pressure in the entire organism and is probably involved in the development of arteriosclerosis. In addition, it is important for the function of NO that its biological half-life is shorter than one second. Therewith, NO can only reach the cells in the immediate neighborhood of the location of formation, i.e. the effect of NO is locally limited.
At least three different NOS isozymes belong to the family of NOS: the endothelial enzyme (eNOS), the brain enzyme (bNOS) and the inducible NOS (iNOS). All three isozymes are isolated by now, their primary structure, i.e. amino acid sequence, solved and the coding gene segments characterized. The essential difference between these NOS lies in their molecular weight and above all in the regulation of the expression and the enzymatic activity. Thus, eNOS and bNOS are regulated through their activity and i-NOS chiefly through expression.
For the characterization of the NOS enzymes the following is illustrated:
eNOS: this enzyme has a molecular weight of 133 kDa, has a binding site for calmodulin which is dependent on free Ca.sup.++ concentration and is present to more than 90% as membrane bound.
bNOS: the brain enzyme is a homodimer with a molecular weight of 160 kDa per subunit which is present at less than 10% as membrane bound. As with the eNOS, the calmodulin binding is dependent on free Ca.sup.++, i.e. both enzymes are only active when the intracellular Ca.sup.++ concentration is increased, for example as a result of a receptor mediated Ca.sup.++ influx.
iNOS: the inducible NOS is also a homodimer with a molecular weight of 130 kDa per subunit. The essential difference to the other isozymes is that the activity of iNOS is independent from calmodulin and therewith independent from cellular calcium. Because the turnover rate for L-arginine for iNOS is approximately 10-100 times higher than by eNOS and bNOS, INOS is also referred to as "high output" NOS. Under basal conditions, iNOS can not normally be detected; it is however strongly expressed after immunological activation through inflammation mediators and endotoxins.